In a friction engagement device 30, as shown in FIG. 3, an internally-toothed disk 34 splined to a hub 32 and an externally-toothed disk 38 splined to a drum 36 are alternately arranged. A friction material 40 is secured on both sides of the internally-toothed disk 34. The internally-toothed disk 34 and externally-toothed disk 38 are engaged to perform torque transmission by the axial movement of a hydraulically-actuated piston 42.
In this type of friction engagement device 30, there occurs friction between the surface of the friction material 40 and the externally-toothed disk 38, producing high heat in these disk surfaces of engagement. Since the heat occurring in the disk engagement surfaces moves mostly to the externally-toothed disk 38 side, the externally-toothed disk 38 is increased in thickness for the purpose of increasing the thermal capacity thereof, thereby improving heat dissipation performance at the friction engagement device 30. However, if the thickness of the externally-toothed disk 38 is increased in an attempt to improve the heat dissipation performance as stated above, the axial length of the friction engagement device 30 also increases, resulting in a failure to meet demands for reducing size and weight.
FIG. 4 shows an example of an arrangement of an internally-toothed disk and an externally-toothed disk in another prior art friction engagement device 50, which was intended to obviate the drawbacks of the friction engagement device shown in FIG. 3. The friction engagement device 50 is provided with a so called single-faced internally-toothed disk 52 and a single-faced externally-toothed disk 54. The internally-toothed disk 52 and the externally-toothed disk 54 are so arranged as to engage with friction material 56. Heat generated in the surfaces of engagement can disperse into the internally-toothed disk 52 and the externally-toothed disk 54. Since the heat can disperse into the internally-toothed disk 52 and the externally-toothed disk 54, it is unnecessary to increase the thickness of the disks 52 and 54. Therefore, it is possible to decrease the axial length of the friction engagement device 50 while maintaining as high a heat dissipation performance as the friction engagement device shown in FIG. 3.
The present inventors have further conducted research into the principle of heat generation and the routes of heat transmission in the prior art single-faced friction engagement device mentioned above, finding that the prior art friction engagement device shown in FIG. 4 still had room for improvement.
First, the inventors noticed that the quantity of heat generated in the surface of engagement increases as it goes outward in the radial direction. That is, since it is believed that a sliding stroke at the time of friction generation is longer as the surface extends radially outward and the force exerted by piston 58 is constant over the entire surface of engagement, the quantity of heat generated in the surface of engagement increases as the surface extends radially outward.
Next, a study was conducted of the routes of heat transmission in the friction engagement device 50. The heat is dissipated to oil or transmitted to a hub 60 and a drum 62 through the internally-toothed disk 52 and the externally-toothed disk 54. The quantity of heat generated in the surface of engagement, however, increases radially outwardly along the surface as described above. The heat generated at the externally-toothed disk 54 is likely to escape to the drum 62. However, the heat generated at the outer edge of the internally-toothed disk 52 is stored in the vicinity of the outer edge because of the presence of a long distance between the outer edge and the hub 60.
FIG. 5 shows the temperature distribution of the internally-toothed disk 52 and the externally-toothed disk 54 in the friction engagement device of FIG. 4. As is clear from this drawing, the internally-toothed disk 52 is at a considerably higher temperature in the vicinity of the outer edge than the externally-toothed disk 54. This indicates that the internally-toothed disk 52 is used less advantageously than the externally-toothed disk 54.
Noticing the principle of heat generation and the routes of heat transmission in a single-faced friction engagement device, the present invention has as an object to equalize the temperature at the frictional surface of this type of friction engagement device.
In other words, the present invention has as an object to decrease the axial length of the friction engagement device while maintaining the heat dissipation performance (heat resistance) of the prior art device.